The use of silicon dioxide as a reinforcing filler in compounds for tyres, in particular for treads, has been known for some time.
Silicon dioxide is used to partially or totally substitute carbon black due to its advantages in terms of rolling resistance and wet grip. The use of silicon dioxide entails a series of drawbacks due to the difficulty of dispersion of the silicon dioxide in the polymer base. This problem derives from the presence on the silicon dioxide of surface silane groups, which favour the formation of hydrogen bonds and, therefore, silicon dioxide particle agglomerates, and give the silicon dioxide hydrophilic characteristics which have a low level of compatibility with the hydrophobic characteristics of the rubber. Poor dispersion of the silicon dioxide in the rubber compound causes extreme variability and heterogeneity of the physical-mechanical characteristics of the compound and, in particular, low resistance to wear. In order to solve the dispersion problems described above, the use of silane bonding agents has been known for some time; said agents inhibit the formation of the above-mentioned hydrogen bonds by bonding with the silanol groups and, at the same time, they chemically bond the silicon dioxide to the polymer base.
Furthermore, the use silicon dioxide requires the presence of specific plasticizers in the compound to favour the processability of the compound.
The need was felt for a solution that favoured improvement of the rolling resistance and wear resistance of the compounds containing silicon dioxide and which, at the same time, was favourable to the processability of the compound thus also avoiding the use of plasticizers.
Lignin is one of the natural polymers that form wood, of which it constitutes 33% by weight. Lignin is insoluble in apolar and organic solvents, and it can be separated from the cellulose by means of a sulphonation process. The lignin is solubilised as calcium salt or sodium salt of lignin sulfonic acid, while the cellulose remains unchanged and, once separated, is used in the production of paper or other cellulosic articles.
Studies of solutions for the use of lignin as a by-product of wood in the production of paper have been ongoing for some time now. In said regard it must be specified that disposal of the lignin represents a limiting stage in the paper production chain.